The present invention relates to a method for purifying 1,1,1,3,3-pentafluoropropane (hereinafter referred to as "HFC-245fa") which is useful as a refrigerant, a blowing agent for preparing rigid urethane foam, and the like, particularly to a method for purifying a crude HFC-245fa containing 1-chloro-3,3,3-trifluoro-trans-1-propene and the like, which are to be removed therefrom.
Today, 1,1-dichloro-1-fluoroethane is used as a blowing agent for preparing rigid urethane foam, and the like. This compound has a capability to destroy the ozone layer, even though its capability is low. Therefore, it is considered as a transient alternative substance and thus is designated as an object which will be limited in use in the future. In view of this, much attention is attracted to HFC-245fa as being an alternative to 1,1-dichloro-1-fluoroethane, since HFC-245fa does not have a capability to destroy the ozone layer.
There is disclosed in U.S. Pat. No. 5,574,192 a method for producing HFC-245fa by fluorinating 1,1,1,3,3-pentachloropropane with hydrogen fluoride in the liquid phase in the presence of a fluorination catalyst of antimony pentachloride. In case that this fluorination is conducted continuously, an intermediate of this fluorination, 1-chloro-3,3,3-trifluoro-1-propene (hereinafter referred to as HCFC-1233zd), will inevitably be distilled off together with HFC-245fa, since these compounds are close in boiling point. This intermediate, HCFC-1233zd, has cis-trans isomers (HCFC-1233zd(c) and HCFC-1233zd(t)). When 1,1,1,3,3-pentachloropropane or HCFC-1233zd is reacted in the gas phase with hydrogen fluoride in the presence of a chromium catalyst and the like, the reaction product may contain a considerable amount of HCFC-1233zd.
In general, distillation is used for separating a liquid or gaseous liquefiable mixture. It is, however, very difficult to separate components of this mixture, if they are close in boiling point. For example, the boiling points of HFC-245fa and HCFC-1233zd(t) are very close, 15.3.degree. C. for HFC-245fa and 21.0.degree. C. for HCFC-1233zd(t). In a normal distillation of a mixture containing HFC-245fa and HCFC-1233zd(t), HCFC-1233zd(t) is distilled off, prior to HFC-245fa. Thus, the relative volatility of these components is assumed to be close to 1. It is known to use extractive distillation in order to effectively separate a mixture containing components having a relative volatility close to 1. There is disclosed in U.S. Pat. No. 5,087,329 the use of 1,2-dichlorotetrafluoroethane as an extractive agent, in an extractive distillation for separating pentafluoroethane from chloropentafluoroethane. Furthermore, there is disclosed in Japanese Patent Unexamined Publication JP-A-7-133240 the use of an extractive agent selected from paraffines (alkanes), alcohols, ethers and ketones, which have standard boiling points under atmospheric pressure within a range of from -10 to 100.degree. C., in an extractive distillation for separating pentafluoroethane from chloropentafluoroethane. Still furthermore, there is disclosed in JP-A-9-12487 the use of an extractive agent selected from carbon chlorides and chlorohydrocarbons, having 1-2 carbon atoms in the molecule, in an extractive distillation for separating pentafluoroethane from 1,1,1-trifluoroethane.